Structure of the atom

Physics · Nuclear and quantum physics. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Geiger and Marsden fired alpha particles at a very thin gold foil. Most went straight through, so most of the atom is empty space. A very small number were deflected through large angles, so the positive charge and almost all the mass are in a tiny nucleus. Electrons move around it.

A nuclide is written with the nucleon number A on top and the proton number Z below. Neutrons = A − Z.

Electrons in an atom can have only certain discrete energy levels. When an electron drops to a lower level it emits one photon whose energy equals the difference between the levels. This is why each element gives its own line spectrum.

Rules to remember

Common mistake

Students put the energy difference into E = hc/λ while it is still in eV. Convert to joules first by multiplying by 1.60 × 10−19.

Worked example

An electron in a hydrogen atom falls from the level at −1.5 eV to the level at −3.4 eV. What is the wavelength of the emitted photon? (h = 6.63 × 10−34 J s, c = 3.00 × 108 m s−1, e = 1.60 × 10−19 C)

  1. Energy difference: ΔE = −1.5 − (−3.4) = 1.9 eV.
  2. In joules: 1.9 × 1.60 × 10−19 = 3.04 × 10−19 J.
  3. hc = 6.63 × 10−34 × 3.00 × 108 = 1.99 × 10−25 J m.
  4. λ = hc ÷ ΔE = (1.99 × 10−25) ÷ (3.04 × 10−19) = 6.5 × 10−7 m.

Answer: 6.5 × 10−7 m (about 650 nm, red light)

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